1. Field of the Invention
The present invention relates to a process for the production of diamond abrasive grains with a high yield which have a relatively uniform size distribution, a high toughness and a high strength.
2. Description of the Related Art
Diamond abrasive grains are bonded by a metal or a resin and used for a grinding wheel, a cutter or a drill. In addition, they are nowadays used as a raw material of a diamond sintered tool for cutting for example aluminum alloy materials.
One process for the production of the diamond abrasive grains is a stacking process. In this process, a graphite plate as a raw material is stacked on a solvent metal plate which comprises at least one of Fe, Ni, Co and Mn to form a production system for the diamond abrasive grains, and then the system is kept under a condition of a temperature and a pressure in which diamond is thermodynamically stable so as to spontaneously generate nuclei at an interface between the graphite plate and the solvent metal plate and grow the nuclei to a desired size.
In the above stacking process, since the spontaneous nuclei are generated randomly with respect to a space and a time, the produced diamond grains have a wide distribution in their size. The resultant diamond grains should be sieved to select diamond grains having a size of 300-600 .mu.m for the use as the abrasive grains. However, a yield of such suitable diamond grains is extremely small and thus, the process is not economically advantageous. In addition, due to the spontaneous nucleation, diamond grains growing from the nuclei interferes with one another resulting in generation of defects in the diamond grains, whereby it is very difficult to produce good diamond grains having a high toughness and a high strength.
In order to overcome the above problem of the stacking process, it has been proposed to provide diamond crystal seeds beforehand between the graphite raw plate and the metal solvent plate. For example, Japanese Patent Kokai Publication No. 68395/1986 discloses a production system in which diamond crystal seeds are placed in small holes regularly provided on a solvent metal plate, and a graphite raw plate is stacked on the metal plate. In the stacking process using the diamond crystal seeds, diamond grains having a desired size are produced in a higher yield than the usual stacking process in which the diamond crystal seeds are not used.
In the process using the diamond crystal seeds, it is difficult for a melted solvent metal to completely surround surfaces of the diamond crystal seeds in an initial term of crystal growth, and therefore many crystal seeds grow while the solvent metal and/or the graphite are included therein, so that the resultant diamond abrasive grains contain the inclusion. Such diamond grains which contain much inclusion are easily cracked because of a difference of a thermal expansion coefficient between the inclusion and the diamond crystal. Therefore, the diamond abrasive grains produced by the stacking process with the diamond crystal seeds have a less mechanical strength, especially a less impact strength than that of the diamond abrasive grains produced by the stacking process without the diamond crystal seeds.
In addition, in the stacking process with the diamond crystal seeds, during the system is kept under a condition in which diamond is thermodynamically stable, not only the diamond crystal seeds grow, but also spontaneous nuclei are generated as in the case of the usual stacking process without the diamond crystal seeds. Thus, these two kinds of crystals interfere with one another so that the produced diamond abrasive grains contain a lot of crystals of a low quality such as crystals having defect and crystals of intergrowth in which more than one crystals are bonded.
Since the spontaneous nucleation is accelerated when a distance between the arranged diamond crystal seeds are longer, it is desirable that the diamond crystal seeds are arranged regularly with a proper distance in order to prevent the spontaneous nucleation. However, the diamond crystal seeds tend to easily roll out of the holes during the construction of the production system, and thus keeping the diamond crystal seeds in position is difficult.